This sounds like it could be tracheomalacia (floppiness of the trachea). Does he tend to NOT cough? Like he has a sign of being sick, but never coughs? The rattle could be from pneumonia, but it would have to be viral, and if he hasn't appeared to be sick in the last 2 weeks, then that can't be it. Our son rattles when he has been sick, and has moderate tracheomalacia secondary to a vascular ring. My friend had a baby with TM. If it doesn't seem to affect his breathing, then it is probably very mild, and it will heal on its own. If he seems to have breathing issues, and this would tend to be during activity, crying, or dream sleep, and then it goes away once napping or calm (but just takes longer than most babies), then I would suspect TM. It could be laryngomalacia instead, but surely he would have had problems before now with that. And, the good news is that it will be resolved by 9-12 mos. 6-8 mos is the worst for primary TM.
The other idea is that if it correlates with eating. If he has an esophagus problem, like a floppy esophagus because of being born early or something or family history, that may cause the rattling. But, that is usually after starting solids, and I wouldn't expect by 7 mos, you would be giving him many finger foods. If he is eating crackers already, then rattling, perhaps that is the problem, and just backing off on the foods would be the answer to give it time to heal.